1. Field of the Invention
The current invention relates to loudspeakers. More particularly it relates to electrostatic loudspeakers.
2. Description of the Prior Art
The majority of loudspeakers in use nowadays are of the dynamic type. The conventional dynamic loudspeaker utilizes an electromagnetic transducer system, often called the motor, to convert the electrical audio signal into a mechanical force. The electromagnetic transducer is then coupled to a diaphragm, in most cases in the form of a cone, which converts the mechanical force into an air vibration, perceived as sound. In other loudspeaker constructions the motor consists of a piezo electric transducer that drives the diaphragm.
Another type of loudspeaker is the electrostatic loudspeaker, where the current invention relates to. Electrostatic loudspeakers have long been acknowledged for their excellent sound quality and lack of audible distortions and colorations. Due to some practical limitations, the electrostatic loudspeaker has commercially never been widely adopted. Some of the aforementioned practical limitations are: a limited sound pressure, electrical safety, a narrow dispersion of the sound wave and a natural roll off at lower frequency as a result of an acoustical short circuit between the front and back of the loudspeaker. Many possible solutions to these limitations have been presented in the prior art. The current invention specifically addresses the limitations in sound pressure and electrical safety.
Contrary to a conventional dynamic loudspeaker an electrostatic loudspeaker does not include a motor and means to transfer the mechanical energy from the motor to the diaphragm. Instead, the diaphragm of an electrostatic loudspeaker is directly driven by the electric field of the audio signal over its entire surface area. Consequently no colorations and distortions associated with a motor or a mechanical energy transfer means exists in electrostatic loudspeakers. Furthermore, since the diaphragm of a dynamic loudspeaker is driven by the motor on only a single point or a limited area, the diaphragm needs to be mechanically stiff to achieve a substantially consistent excursion of the diaphragm over its entire surface area. The diaphragm of a conventional dynamic loudspeaker is therefore most commonly constructed in the form of a cone or a dome and has a significant thickness to provide the required stiffness. Since the diaphragm of an electrostatic loudspeaker is driven over its entire surface area, mechanical stiffness is not required and the diaphragm can be made as thin as practically feasible. As a consequence of the very thin diaphragm together with the absence of a motor and mechanical energy transfer means, the total moving mass in an electrostatic loudspeaker is orders of magnitudes less than that of a dynamic loudspeaker, which will further reduce distortion.
FIG. 1A illustrates a typical electrostatic loudspeaker construction. The electrostatic loudspeaker (100) comprising of first and second stators (102, 104) a diaphragm (103) and several spacers (101). Said stators consisting of a rigid electrically conductive material with a substantially equal relative open surface area in order to pass sound wave through substantially unimpeded, established by for instance a homogenous perforation pattern over the entire surface area of said stators. The diaphragm (103) consists of a thin flexible film of sufficient electrical conductivity to evenly distribute an electrical charge over its surface. The aforesaid stators (102, 104) and diaphragm (103) can come in a multitude of shapes and forms, including, flat surfaces, curved surfaces and rectangular or substantially circular shapes. The electrostatic loudspeaker panel further includes one or several insulating spacers (101) to provide a substantially constant distance between the stators and the diaphragm over the surface of the panel.
FIG. 1B illustrates a schematic representation of the electrostatic loudspeaker (100) of FIG. 1A including a driving circuit as it is used in the majority of the prior art of electrostatic loudspeakers. When no audio signal is present, both stators (102, 104) are at ground potential. The diaphragm (103) is charged up through resistor (112) by DC voltage source (125). The DC source can have either a positive or negative polarity. Typical potentials for the DC source are between 1000V and 6000V. When the diaphragm is positioned in the middle of the stators, the electrostatic forces between the diaphragm and the front and back stators are equal in force but opposite in direction, consequently cancelling out on the diaphragm. The diaphragm will therefore stay in its centered position. An AC audio signal coming from audio source (135) is applied to the stators (102, 104) by means of a step up transformer (130) with a center tap on its secondary winding which is connected to ground. The AC audio signal from audio source (135) will now appear in opposite phase on the front and back stators of the electrostatic loudspeaker (100) resulting in an AC electric field between the stators proportional to the audio signal from said audio source. The force and excursion of the diaphragm as a result of the AC electric field will produce an audio wave emitting from the diaphragm (103), perpendicular to said diaphragm. Because of the symmetric construction of the electrostatic loudspeaker (100) sound is emitted with substantially equal amplitude from both sides of the diaphragm, but opposite in phase. The electrostatic loudspeakers from FIG. 1 are therefore a dipole loudspeaker.
An alternative driving method for the electrostatic loudspeaker (100) as described above is taught by Smith et al. in U.S. Pat. No. 7,054,456 illustrated in schematic form in FIG. 1C. This invertedly driven electrostatic speaker uses a fixed electrostatic field between the stators (102, 104) generated by DC voltage sources (125) and (126). The high voltage audio source (135) is driving the diaphragm without the need of a balancing transformer. A disadvantage of the electrostatic loudspeaker as described in U.S. Pat. No. 7,054,456 is the need for a diaphragm with a low surface resistance in order to avoid electrical loss of the audio signal in the resistance of said diaphragm. A further disadvantage of said electrostatic loudspeaker is that the front and back stators are connected to high DC voltages, substantially compromising electrical safety.
An electrostatic loudspeaker with enhanced sound pressure is presented by Maeda in U.S. Pat. No. 5,471,540 illustrated in schematic form in FIG. 2. An electrostatic loudspeaker (200) includes three diaphragms and four stators, a first and second DC bias power supply (225,226) of substantially equal but opposite voltage. The front stator (202) and all odd number stators (206) are connected to one side of the balancing transformer (230) and all even number stators (204,208) are connected to the other side of said balancing transformer. In reference to each individual diaphragm, the stators on each side of the individual diaphragms carry an audio AC voltage proportional to audio source (235), equal in amplitude but opposite in phase. Furthermore diaphragms 203 and 207 have substantially equal but opposite DC bias voltages in comparison to the DC bias voltages on diaphragm 205. As a result of the opposite polarity of the electrical fields caused by the AC voltage from the audio source (235) and the balancing transformer (230) between the different stators and the opposite DC bias voltages on the diaphragms in between the different stators as described above, the electrostatic forces on all diaphragms as a result of the momentary voltage of the audio source (235) and therefore the excursions of all diaphragms occur in the same direction. Consequently the acoustic energy radiating from each diaphragm will be added substantially. Disadvantages of the electrostatic loudspeakers described by U.S. Pat. No. 5,471,540 are an unbalance of the static forces on the diaphragms and a limited ability to reproduce higher frequencies. A further disadvantage of said electrostatic loudspeaker is that the front and back stators are connected to high AC voltages, substantially compromising electrical safety.